1. Field of the invention
The present invention relates to a camera accessory, and more particularly to a camera accessory that can be used by replacing the back cover of a camera body.
2.Description of the Prior Art
There are known various attachments attachable in front of the ordinary objective lens having a fixed focal length, in lens shutter cameras, in order to allow the camera to take a picture at a longer focal length than the focal length of the objective lens. However, these attachments are very poor in optical performance and make it difficult to take pictures with satisfactory optical performance.
Cameras with an objective lens system having two selectable focal lengths are also proposed in recent years. However, such cameras are larger in size and more expensive than the cameras with objective lenses having fixed focal length.
Single-lens reflex cameras with lens interchangeable type may be used with an accessory such as a tele-converter which can be mounted between the objective lens and the camera body for extending the focal length of the photographing optical system composed of the objective lens and the tele-converter 1.4 or 2 times greater than that of the objective lens only. Such an accessory is also large in size and costly, and is further disadvantageous in that it lowers the performance of the photographing optical system when mounted. When the tele-converter is mounted and automatic focus adjustment is to be made of the photographing optical system by the motor in the camera body, many problems are required to be solved since the relationship between the driving amount produced by the motor and the shifting amount of the in-focus position of the photographing optical system corresponding to the driving amount becomes different by the tele-converter from that when the objective lens is only used.
Japanese Laid-Open Patent Publication No. 54-26721 discloses a camera proposed to solve the aforesaid problems of the conventional arrangements. The disclosed camera is selectively switchable into a real focal length photographing mode and a pseudo focal length photographing mode in which a printed area is smaller than that in the real focal length photographing mode. When the pseudo focal length photographing mode is selected, a printed range is indicated by the viewfinder, and information indicative of the pseudo focal length photographing mode is recorded on a film frame when it is exposed. Upon reading the information, a printer enlarges a portion (trimmed range) of the film frame and produces a print similar to one that would be produced if the focal length of the objective lens were extended.
With this arrangement, the camera is not increased in size and is inexpensive, and the performance of its photographing optical system is not lowered. The automatic focus adjustment can be effected, without any problem, in the same manner as in the real focal length photographing mode.
However, the disclosed proposal requires the production of a special camera dedicated to the aforesaid intended modes, and hence no pictures can be taken in the pseudo focal length photographing mode by commercially available cameras.